I don't grasp the boldened sentences. If a member of the House of Lords last held office as Prime Minister in 1902, then how can't we "conclude that this has always been the case since 1902"? Doesn't "such an occurrence is impossible today" prove this has always been the case since 1902?

Pls see the question in the title. To tell whether practice on a certain matter has hardened into a rule, don't you just regard the duration of the practice? 1708 was the last time a monarch (Queen Anne) refused assent. 1965 was the last time a non-member of either House of Parliament entered the Commons at an early by-election, "since no government today willingly causes a
by-election
to be held in one of its own seats" (p 21). Thus royal assent has lasted longer and is more of a rule than the principle of
ministerial responsibility that a minister belong to a House of
Parliament.

So what if "the practice is negative in character"? You can frame the above two Constitutional Conventions as negative practices:

"Queen may not of her own initiative refuse the royal assent" to a Bill passed by both Houses of Parliament.

"There is no rule of law which
prevents the monarch appointing to ministerial office a person who is outside Parliament." (p 21)

General characteristics138

The starting point for whether or not a convention exists is usually the tripartite test proposed
by Sir Ivor Jennings: ‘first, what are the precedents; secondly, did the actors in the
precedents believe that they were bound by a rule; and thirdly, is there a reason for the
rule?’139 This approach is, of course, contested; certainly all of the stages of the test contain
important ambiguities or disguise value judgments.140 It is highly unlikely that all conventions
require all limbs of the test to be satisfied, at least not in the same way or to the same
extent: the Sewel convention noted above, for example, came into being effectively immediately
without the need for historic precedent.141 Other long-established conventional rules
have great authority and are universally known. Many have developed out of a desire to avoid
the formality, explicitness and publicity associated with changes in the law. The role of the
monarch in the conduct of government has almost disappeared since the 18th century without
a series of statutes removing one royal power after another. In the same way, powers have
been acquired by the Prime Minister by operation of convention rather than as the result of
legislation. Conventional rules may be used for discreetly managing the internal relationships
of government while the outward legal form is left intact.
The informality of such rules is often accentuated by the fact that the rules themselves are
not formulated in writing, but this is not always the case. As we have seen, the rule that judges
should not undertake political activities is now in a written form. In 2011, the government

p 24.

issued The Cabinet Manual, which derived from a similar document prepared previously in
New Zealand, as ‘a guide to laws, customs and rules on the operation of government’.142 The
booklet covers all the machinery of central government in an informative way, but it is essentially
descriptive; it does not claim any special authority (it has not been approved by Parliament),
and the government reserves the right to change the practices that are described. In
reality, some of the practices are too well-established to be changed (for instance, the rule that
the Queen’s speech in opening a session of Parliament is written by the government).
The development of unwritten rules is often an evolutionary process that occurs before
clear rules of conduct emerge. In retrospect, we can identify when (for instance) a member of
the House of Lords last held office as Prime Minister (1902). But because such an occurrence
is impossible today, we cannot conclude that this has always been the case since 1902. At any
given time, it may be difficult to tell whether practice on a certain matter has hardened into a
rule, particularly when the practice is negative in character. Before the Constitutional Reform
Act 2005, which took away the judicial role of the Lord Chancellor, there was uncertainty
over the extent to which the Lord Chancellor, as a government minister, might properly sit
as a judge to decide appeals in the House of Lords.143
As with all forms of rules, disputes may arise about the meaning and effect of conventional
rules, particularly when they have no definitive written form. The enforcement of many conventions
depends essentially on the force of public and political opinion. If many legal rules
have an ‘open texture’,144 how much more ‘open’ will be the texture of non-legal rules where
there is no definite procedure for resolving disputes about their existence and content. The
vaguer the convention – perhaps better expressed as a principle than a rule – the harder it
will be to show consensus as to its boundaries, unless there is a single authoritative figure who
can enforce it (such as the Prime Minister with the Ministerial Code).

1 Answer
1

The author observes that there is a current constitutional rule that "Prime Ministers must be a member of the Commons." But "This rule was not in force in 1902".

But because this rule is based on convention, we can't say the date on which this rule was introduced. It probably wasn't a rule in 1903, it probably was a rule by 1970. I don't know if it was a rule in 1930. This is in contrast to a written rule in a documentary constitution or a statue law. We know the exact date of the 23rd Amendment in the USA or the Parliament Act in the UK.

When discussing other acts it can be hard to decide if there is a rule or not. Can a PM prorogue parliament for political benefit? Can a PM evade the requirements of the Fixed-term parliament act by passing an amendment by a simple majority? Can the PM wear ripped Jeans in the chamber? Is a bearded Prime Minister permitted?

The beard question is pertinent since no PM has worn a beard since 1902.

The last bearded PM was Robert Cecil, for anyone else who feels like looking it up.
– bobsburnerFeb 18 at 15:33

Robert Gascoyne-Cecil, and he was also the last PM not to be an MP (he was Lord Salisbury)
– James KFeb 18 at 15:39

There was no woman prime minister between 1902 and Margaret Thatcher. That didn't prevent her election. Surely there is an inference to be drawn here on the question of bearded prime ministers.
– phoogFeb 18 at 16:06

3

I think you may be missing the point... which is that you can't infer a constitutional prohibition only from the absence of something. You can't infer that a PM can't be a Lord only because no PM has since 1902. Much stronger evidence is the actions fo Douglas-Home in renouncing his peerage. Moreover it may be impossible to decide when the rule on Lords being PM was introduced. Such is the nature of an unwritten constitution.
– James KFeb 18 at 19:46